This invention relates generally to data input devices and particularly to touch-sensitive apparatus used in conjunction with computer data display screens.
Interactive computer systems usually require an operator to observe data displays on a cathode ray tube or other display screen and to enter data on a keyboard. While keyboards are suitable for entry of large amounts of character-oriented data, they have disadvantages in many applications--especially those contemplated for microcomputers. Keyboards require training for efficient use and many microcomputer users have not had such training, nor are they inclined to take it. Moreover, with a keyboard, the data display and data entry functions are separated both spatially and conceptually. It is sometimes difficult for an operator to associate display and input functions in a natural way when they are separated by such a physical and perceptual distance.
Touch-sensitive display screens have been proposed to alleviate the problems of using keyboards for data entry in some applications. Typically such screens consist of a plurality of light emitters and detectors arranged around the sides of the display screen so that they cover the display surface with crossed light beams. Touching the display surface with a finger or pointer breaks one or more of the beam allowing the apparatus to calculate the position of the touch. In an interactive computer system, therefore, the computer can display information on the screen and the operator can make selections, confirm data or otherwise input data by touching the appropriate point on the screen.
Such an apparatus requires carefully collimated light beams in order to provide unambiguous position measurement. Such collimation requires bulky optical arrangements that limited acceptance of light beam touch screens. To eliminate the requirement for collimation, only a few, widely separated light beams in each orthogonal direction are energized at a time. This is done by enabling the emitters and detectors by pairs in sequence. If this is done quickly enough, the effect is the same as if a grid of beams covers the screen continuously.
In this type of apparatus, accurate position sensing depends on the accurate and unchanging spatial relationships between emitters, detectors and the display screen. Once the apparatus has been calibrated, the user must be confident that these relationships cannot change significantly over time. This typically has meant that the touch detecting apparatus must be built into the display screen housing. While this is satisfactory in many applications, users who already have display screens without touch-sensitive input capability cannot have the benefit of such data input means without replacing their display screens. In addition, the touch-sensitive input apparatus cannot be removed easily for cleaning and maintenance.
Further, to maintain the alignment of the emitters and detectors, they have been rigidly attached to a sheet of transparent material which is mounted in front of the display screen and through which the beams propagate. While this avoids the problem of misalignment, it does so at the cost of interposing a material between the display screen and then observed with the consequent possible loss of brightness and contrast.